More Diffraction-limited Images at the KPNO 4-m Telescope (1Dec95)
(from NOAO HIGHLIGHTS!, NOAO Newsletter No. 44, December 1995)
During the summer shutdown in 1994 we conducted an experiment to
determine the parameters appropriate for infrared shift-and-add imaging
at the KPNO 4-m telescope. As reported in an earlier Newsletter, we
built a new camera for the Cryogenic Optical Bench (COB) with a scale
of 0.1" per pixel, and took bursts of data at rates up to twenty frames
per second. We found that we could often achieve images with a full
width at half maximum of 0.3" at wavelengths longer than 3 um with
frame rates of only a few hertz. To capitalize on this finding from the
prototype experiment, we built a data system that can perform the shift
and add calculations in real time. During the summer shutdown this year
we installed and successfully operated this system. In early
September, this diffraction limited infrared imaging experiment
(DLIRIM) was operated in service mode, producing satisfactory data for
several proposers.
In each of the illustrations shown here the field of view is
approximately 20". (We used a 256 x 256 InSb detector, and the images
have been cropped after shift and add processing).
This experiment was built by Jerry Heim and Nick Buchholz, and operated
by Mike Merrill and Ian Gatley. The Lucy deconvolution is by Tod Lauer.
[Photos not included]
Figure 1: The left panel shows the Galactic center in the
L band. The point spread function can be seen to be uniform
across the field. A Lucy deconvolution of this image is
shown at right.
Figure 2: This shows an image of the H II region W3A in the L
band with one taken at the 50 inch telescope (0.9" per pixel)
through a narrowband 3.3 um filter centered on the wavelength
of a bright dust emission feature. The bandpass of the
narrowband filter is contained within the L filter used at the
4-meter telescope. It was necessary to use the wider filter at
the 4-m because of the requirement in shift and add mode that a
star in the field be detectable in each individual frame. The
limiting magnitude for such stars in this experiment is about
L = 8.
Figure 3: The Galactic center in Brackett a emission. This image
was made by differencing two images taken in narrowband (1%)
filters on and off the emission line, using the same star for
shift and add in both cases. The shift and add algorithm employs
integer pixel shifts, and returns the (integer) address of the
star used for shifting, making the differencing of the two
images easy. Notice the diffraction pattern around the emission
line star in the lower right.
Figure 4: The bright reflection nebula in W75N. Earlier infrared
images had left unresolved the issue of whether this object was
a star. This snapshot clearly shows its extent.
Ian Gatley
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